Air-drying alkyd compositions have many useful applications in various types of coatings. Examples of coatings include inks, paints, resins, and surface coatings such as linoleum. Alkyd compositions contain polymers formed from the reaction of an unsaturated oil or unsaturated fatty acid, polyalcohol(s) and polyacids (or corresponding anhydrides, and usually one or more carrier solvents for the polymers. Alkyds are typically applied as a liquid coating onto a surface or substrate. The coating oxidizes upon exposure to air, eventually forming a solidified coating on the surface. Ambient cure alkyd compositions can air dry to its solidified form at ambient temperature (i.e. without the addition of heat).
Oxidative air drying of an alkyd composition is due to autoxidation and cross-linking of the unsaturated oil/fatty acid component of the alkyd composition, and simultaneous evaporation of the carrier solvent(s). Absorption of oxygen from the air causes peroxide formation and peroxide decomposition, which results in the generation of free radicals (see Scheme 1(a) and (b) below) (Bieleman, J. and Lomolder, R. “Chapter 7: Catalytically Active Additives” in Additives for Coatings, J. Bieleman (ed.) Wiley-VCH (2000)). The free radicals initiate cross-linking and formation of higher molecular weight polymers, eventually leading to a solidified “air dried” film or coating.


The time for an alkyd composition to dry depends on the concentration and the type of unsaturated oil used to prepare the alkyd composition. Autoxidation and crosslinking of the unsaturated oil/fatty acid component can proceed unaided, but the time for drying is generally found to be unacceptably long. The reactions are significantly accelerated by the presence of a metal-based drying catalyst, commonly referred to as a “drier”. Without the presence of a drying catalyst, the alkyd coating would likely take a number of months to dry. In the presence of a drying catalyst, drying can be accomplished within a few hours. The metal within the drying catalyst catalyzes autoxidation by forming a complex with both atmospheric oxygen and the double bonds of the unsaturated fatty acid groups within the alkyd composition.
The catalytic activity of the transition metal during decomposition of the hydroperoxide (ROOH in Scheme (b) relies on the repeated transition of the metal ion from the lower to the higher oxidation state and back again, leading to reduction and oxidation of the hydroperoxides catalyze and accelerate oxidation of the unsaturated oil component of the composition. Transition metals are most commonly employed in such driers, as transition metals are capable of undergoing a transition from a lower valence state to a higher valence state in a redox reaction with fatty acid peroxides present in the alkyd composition.
In the past, organic lead salts have been used, but due to their toxicity, lead-based driers have been replaced with driers based on other transition metals such as cobalt, manganese, iron, cerium and vanadium. At present, cobalt carboxylate salts are the most widely used drier in air-drying alkyd coatings. Cobalt-based driers are popular since the drying process is effectively accelerated with low concentrations of cobalt present.
Commonly used transition metal driers are carboxylate salts, having the general formula Mx+[(RCOO)−1]x, wherein M represents the transition metal with valence x and R represents an aliphatic (typically C6-C18) carboxylate group. The carboxylate group stabilizes the transition metal and also allows solubilization and even distribution of the drier throughout the alkyd composition, which typically includes one or more organic solvents to solubilize the various components of the composition. A typical example of such a drier is described in U.S. Pat. No. 5,759,252. Additional examples are described in Bieleman, J. and Lomolder, R., “Chapter 7: Catalytically Active Additives” in Additives for Coatings, J. Bieleman (ed.), Wiley-VCH (2000).
Commercially available driers can consist of an individual primary drier or contain a combination of different driers, with a primary drier responsible for the catalytic activity, and one or more auxiliary driers and/or coordination driers. Auxiliary driers interact with the primary drier. Coordination driers form coordination complexes with hydroxyl groups within the alkyd composition and thus help to stabilize the polymer network of the alkyd composition. Auxiliary and/or coordination driers are typically based on barium, zirconium, calcium, bismuth, zinc, potassium, strontium and lithium. Auxiliary and coordination driers are added to enhance the activity of the primary drier and the final characteristics of the dried coating (e.g. hardness, glossiness).
The metal ion of the primary drier depends upon factors such as activity of the drier at ambient temperature, possible colouring effects (important in paint applications), toxicity, the type of alkyd composition in question, and cost.
In general, commercially available transition metal driers have poor storage stability, particularly upon addition to the compositions to be dried. Upon exposure to water and oxygen in the atmosphere and/or within the composition to be dried, the transition metal tends to change oxidation state, thus losing its catalytic activity over time. As a result, the drying time of the alkyd composition containing the drier tends to increase the longer the alkyd composition is kept on storage, The increase in drying time of the alkyd composition, over time, is referred to as a “loss of dry time stability”.
Alkyd compositions prepared as water-in-oil emulsions or oil-in-water emulsions are particularly prone to the problem of loss of dry time stability, due to deactivation of the drier in the presence of water. In these emulsions, the drier is distributed within the water phase. However, the drier is rapidly deactivated in the water phase, due to complexation with other water-soluble components, such as pigment molecules, and hydrolysis of the metal salts. This also results in an uneven distribution of the drier in the oil phase after evaporation of the water phase.
Cobalt-based driers provide good catalytic activity at ambient temperature and are the most commonly used driers in ambient cure alkyd compositions. Cobalt carboxylate salts are the most commonly used form. However, upon addition to the alkyd composition, the catalytic activity of the cobalt salts decreases over time. Also, cobalt is suspected to be toxic.
Attempts have been made in the past to improve the shortcomings of known transition metal driers. International patent application WO 2003/093384 discloses a drier composition for an air-drying alkyd based coating, comprising a transition metal salt and a reducing biomolecule which is capable of undergoing a transition metal catalyzed oxidation, to prolong the catalytic activity of the transition metal drier. European Patent Application No. 1 382 648 A1 discloses a drier for air drying alkyd based coating, which is based on a transition metal selected from a group that does not include cobalt.
For an acceptable drying time, an alkyd composition containing a transition metal drier should be used soon after preparation and consequent exposure to the atmosphere. This represents an inconvenience to the consumer (i.e. the end user). Also, there can be significant wastage of product that is no longer deemed useable, i.e. the alkyd composition has such a long ambient drying time as to render it unfeasible to use. This poses problems for both the consumer and the manufacturer, as well as the environment.
Accordingly, there is a need for alternative driers for alkyd compositions with improved stability, which can provide better dry time stability in alkyd compositions. There is also a need for less toxic and more environmentally friendly alternatives to currently available driers.